This invention relates to cellular mobile radio-telephony systems and, more particularly, to a filtering operation useful in evaluating variations in amplitude or signal-to-noise ratio or padding etc. of voice frequency (VF) transmissions at a mobile subscriber telephone station traveling in the vicinity of a cell border, thereby to facilitate a hand-off between cells.
Cellular radio-telephony is used at numerous sites in this country and overseas. Such a telephone system permits communication between two mobile telephone stations as well as between a mobile station and a fixed station.
Such systems are formed of clusters of cells wherein each cell is assigned a predetermined set of voice frequency channels, the bands being spaced apart in the frequency spectrum so as to permit simultaneous transmission of many telephone conversations by many stations without interference between communications in the various bands. In order to insure that there is no interference between the assigned frequency bands of one cell and the assigned frequency bands of a contiguous cell, the bands in the contiguous cell are located at different portions of the frequency spectrum than the bands of the first-mentioned cell. The same frequency bands are repeated at more remote cells, and the power of signal transmission in any one band is limited in amplitude so as to become attenuated to a sufficiently low, non-interfering level at the frequency bands of the remote cell.
Cellular mobile radio-telephone systems are described in the literature. One such system referred to as an "Advanced Mobile Phone Service" is described in The Bell System Technical Journal, January 1979, Vol. 58, No. 1, pp 1-269.
Multiplexing of individual subscriber channels for communication via common RF (radio frequency) link is accomplished, preferably, by means of statistical multiplexers. Such multiplexers are described in an article entitled "Controlling Data Communications: Statistical Multiplexer Moves In" by H. J. Hindin in Electronics, July 28, 1981, pp 141-148, and in "A Buyers Guide to Today's Volatile Statistical Multiplexers" by J. H. Scharen-Guivel and A. A. Carlson in Data Communications, March 1982, pp 97-126. A switching configuration for a mobile system is disclosed in "A Distributed Switching Approach to Cellular Coverage" by R. E. Pickett in Telecommunications Magazine, February 1983. A network control system for use in cellular mobile radio-telephony may include the commercially available ITT System 1210 hardware and software.
In the construction of a cellular system, a group of the foregoing cells is clustered about a system switching network or piece of the network which allocates the available frequency bands in any one cell among the various mobile radio-telephones with which communication is desired. Such switching networks provide for the Coupling of a telephone conversation of one frequency band in a first cell with a second frequency band in a second cell or, alternatively, with a long-distance trunk circuit which connects the first cell with a desired cell in another cluster or a land subscriber. In addition, well-known control circuitry is provided for the transmission of command signals to the mobile stations for directing their respective transmissions on the allocated frequency bands.
As a mobile subscriber moves from one cell to the next cell, a hand-off procedure is followed wherein the central switching network commands the mobile station to switch frequency from the band which was used in the first cell to the frequency of a new band to be used in the second cell. While such hand-offs are usually accomplished in an effective manner, a problem arises in that, under certain circumstances, such hand-off procedures can become excessively burdensome to a central computer utilized in the operation of the central switching network.
In particular, the problem is manifested by a variation in the amplitude of signal transmissions with the mobile station at and in the vicinity of the interface between the two contiguous cells. The variation may be characterized by a sequence of pulsations in amplitude or signal-to-noise ratio or padding, rather than a monotonic variation in amplitude. A characteristic in hand-off decision-making circuitry presently in use is the measurement of the amplitude of signal transmission with the mobile station. The central switching network may include directive antennas which designate a specific sector in azimuth which shows generally the position of the mobile station with respect to the cell cluster. Thus, a variation in amplitude of signal transmissions within predetermined amplitude ranges serves as an indication that the mobile transmitter is centrally located within a cell, or is located near a boundary of the cell. Thereby, by monitoring the amplitude of such signal transmissions, the decision-making circuitry is able to signal the central switching network at the appropriate time when a hand-off is to be made from one frequency band to another frequency band.
Under conditions wherein the presence of tall buildings or other typographical features induce pulsations in the otherwise monotonic variation in signal amplitude, the decision-making circuitry receives false alerts as to the proximity of the mobile station to the cell border and, accordingly, proceeds to initiate hand-offs back and forth between the two contiguous cells in accordance with the pulsations sensed in the transmitted signal amplitude. Such activity in the decision-making circuitry places a burden on the computer employed in such circuitry in that a much larger usage of the computer is required for a single hand-off procedure.
During intervals of relatively low traffic volume, such additional burdens on the computer may not degrade the overall number of telephone communications which can be simultaneously handled. However, during intervals of relatively large volumes of telephone traffic, such a burden may limit the amount of telephone communications which can be handled by the computer and the hand-off circuitry of the central switching network. It is further noted that such excessive switching back and forth between contiguous cells interferes with the general plan of channel allocation among the available frequency bands.